1. Field of the Invention
Aspects of the present invention relate to an optical pick-up and a disc apparatus having the same, and more particularly, to an optical pick-up in which inferiority of a tracking signal caused by an adjacent layer during the recording and/or reproducing of an optical disc with multiple recording layers is effectively prevented, and a disc apparatus having the same.
2. Description of the Related Art
Optical discs are being developed and commercialized to record and store high definition image information and high quality audio information for a long period of time. The optical disc is a recording medium in which a very great deal of pits is formed in the surface so as to change reflection of a laser beam to record and/or reproduce information such as voice, images, documents, and the like. Conventional optical discs, such as compact discs (CD), digital versatile discs (DVD), and the like, are chiefly used. Recently, as the conventional disc is reaching a limit in recording capacity, new optical discs, such as a recordable/rewritable blue-ray disc (Blu-ray disc, or BD), an advanced optical disc (AOD, also called HD-DVD), and the like, capable of recording a vast quantity of information (more than a few tens of gigabytes) are being developed and are starting to be widely used.
The capacity of the information recorded in the optical disc is in inverse proportion to a size of a light spot focused on the surface of the optical disc. The size of the light spot S is determined by a wavelength λ of a laser beam to be used and a numerical aperture NA of an objective lens in the form of the following formula 1.S∝kλ/NA  Formula 1
where k is a constant dependent on an optical system and has a value of usually 1 to 2.
Thus, in order to record more information in the optical disc, the size S of the light spot focused on the optical disc must be decreased. Moreover, in order to decrease the size S of the light spot, as expressed by the formula 1, either the wavelength λ of the laser beam must be decreased or the numeric aperture NA must be increased.
In other words, as the capacity of the optical disc increases, a light source with shorter wavelength and an objective lens with higher numeric aperture must be used. For example, near infrared light with a wavelength of 780 nm and an objective lens with a numeric aperture 0.45 are used for compact discs, and red light with a wavelength of 650 nm (or 630 nm) and an objective lens with a numeric aperture of 0.6 (0.65 in a recordable type) are used for digital versatile discs. For Blu-ray optical disc, light with a short wavelength (405 nm to 408 nm), a blue light, and an objective lens with a numeric aperture of 0.85 are used.
Recently, as the quantity of information to be recorded in the high density optical disc (BD) increases, a multilayer optical disc having two or more layers formed in one side or both sides of the optical disc is being developed. In this multilayer optical disc, light returned to an optical detector during the recording and/or the reproducing is influenced by a layer (a layer to be recorded and/or reproduced) positioned at a focal point of the objective lens as well as by adjacent layers. Since a gap between the recording layers defined in the standard is determined within a bound where the information in the optical disc does not influence an interlayer cross-talk, the interlayer cross-talk should not influence a servo-signal.
FIG. 1 shows a light path for the reproduction of the multilayer optical disc in which a focal point of light L10, reflected by a layer L0 (a layer adjacent to a layer to be reproduced, hereinafter referred to as “adjacent layer”), is positioned in front of that of light L11 received by an optical detector 1 during the reproduction of a layer L1 (layer to be reproduced, hereinafter referred to as “reproducing layer”) near to a light incident surface. A focal point of light L01 reflected by the adjacent layer L1 is positioned behind a focal point of light L00 received by the optical detector 1 during the reproduction of the layer L0 (adjacent layer) farther away from the light incident surface.
A differential push-pull (DPP) method is used in the dual layer optical disc to detect a tracking error signal. The light is split into three lights, 0th order light (main light) and ± first order lights (sub-light), using a grating and intensity ratio of the split light. The ratio—first order light: 0th order light:+first order light is equal to or greater than 1:10:1. This is because it is advantageous to use the 0th order light beam by increasing the intensity of the 0th light beam in view of a use effect.
When, in the multilayer optical disc, the DPP method is used to detect the tracking error signal, the 0th order light reflected by the adjacent layer is overlapped to the ±1 order lights reflected by the reproducing layer, so that the tracking signal deteriorates. Since the difference of light intensity between the 0th order light reflected by the reproducing layer and the 0th order light reflected by the adjacent layer is very large, the 0th order light of the adjacent layer does not influence the reproduction signal. However, since the light intensity difference between the ±1 order light reflected by the reproducing layer and the 0th order light (main light) reflected by the adjacent layer is not relatively large, the 0th order light of the adjacent layer has considerable effects on a differential signal (sub-tracking signal) used to detect the tracking signal using the DPP method.
In order to solve the above problem, Korean Patent Unexamined Application Publication No. 2005-0074839 proposes an optical pick-up in which 0th order light (main light) reflected by the adjacent layer is restricted to improve the deterioration of the tracking signal due to the adjacent layer. The disclosed optical pick-up uses a polarized hologram to diffract the 0th order light reflected by the adjacent layer to regions other than a detector region so as to prevent 0th order light reflected by the adjacent layer from entering a sub-photodetector.
However, the polarized hologram blocks the 0th order light reflected by the adjacent layer from entering the sub-photodetector as well as 0th order light (signal light) reflected by the focused reproducing layer (signal layer) and entering a main photodetector. When the 0th order light (signal light) reflected by the reproducing layer enters the main photodetector to detect a radio frequency (RF) signal, since about 10% of the light is blocked by the polarized hologram, the magnitude of a signal to be detected is decreased and signal characteristics, such as the jitter characteristic, deteriorate. Particularly, since the profile of the incident light into the photodetector is a Guassian type, and the architecture of the polarized hologram blocks a central area of the Guassian profile, that is, a part where the intensity of a signal is the highest, the RF signal characteristic is significantly deteriorated.
If the area of the polarized hologram is decreased in order to mitigate the deterioration of this RF signal characteristic, it is difficult to achieve the original purpose of restricting the 0th order light (noise light) reflected by the adjacent layer from entering the sub-photodetector. Furthermore, since the blocking area of the polarized hologram must be increased when light receiving magnification is small, the magnitude of the RF signal is more decreased and the RF signal more deteriorates.
This problem is disclosed in the above patent publication. To solve the problem, sub-photodetectors are added to separately detect lights diffracted to areas separated from that of the photodetector and to compensate the deterioration of the signal characteristic. However, since the optical signals used for the compensation, diffracted to the separated areas and separated from each other, contain signal coherent noise initially, it is not sufficient to help the proper compensation of the RF signal.
Thus, as a practical improvement for the deterioration of the RF signal characteristic, another solution would be to minutely adjust the polarized hologram so as to find a point where the influence by the adjacent layers is minimized and an original signal from the reproducing layer is maximized. However, this improvement causes the number of components for the minute adjustment to increase and increases manufacturing costs and manufacturing time.